1. Field of the Invention
The present invention relates to a semiconductor device manufacturing method, and particularly to a method of manufacturing a semiconductor device having a metal film formed on a gate insulating film.
2. Description of the Background Art
For the miniaturization of semiconductor devices, materials having high dielectric constants (High-k) are adopted to form gate insulating films. Also, there are semiconductor devices in which a metal film is provided on the gate insulating film so as to form gate electrodes. In methods of manufacturing semiconductor devices thus structured, dry etching is performed to partially remove the metal film (patterning).
After the dry etching, etching residues of the metal film remain. Accordingly, it is necessary to perform a cleaning process using, e.g., hydrogen fluoride, in order to remove the etching residues.
A technique for forming a metal film on a gate insulating film having a high dielectric constant is disclosed in Japanese Patent Application Laid-Open No. 2005-72316.
However, applying cleaning process after the dry etching reduces the thickness of the gate insulating film. The thickness of the gate insulating film is reduced also before or after the dry etching. However, the thickness of the gate insulating film is reduced more considerably during the cleaning.
It is thought that this is because the plasma generated during the dry etching damages the gate insulating film lying under the metal film to be removed, and the bonding of molecules of the gate insulating film is weakened in the damaged portions. The reduction of thickness may occur also when the gate insulating film is not a high-dielectric-constant film.
For example, when a cleaning process was performed for about 20 minutes after dry etching, reductions of thickness of about 8 nm to 9 nm (or more) on average were observed with high-dielectric-constant gate insulating films.
The thickness reduction of gate insulating films causes variations of leakage current in the transistors and increases the value of leakage current (that is, it causes deteriorations of electric characteristics of the manufactured transistors). It is therefore desirable to suppress the reduction of thickness of gate insulating films.